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The Scaling mode property is not yet implemented in the intel driver (see here). It is implemented in the proprietary AMD and NVIDIA drivers and since recentlty in the open source radeon driver (here).

However you can work around this limitation by performing the scaling manually with xrandr:

Assume your screen has a native resolution of 1920x1080 and you want to display an unstretched 1024x768 resolution. The proportion of the aspect ratios is 16/9 to 4/3. so 1.333333 to 1.

This puts the screen in the right resolution with the right aspect ratio. But it is not centered. To achieve this we need to use the transform option to put it (roughly) in the center of the screen (1024*1.3333333 - 1024 = 342 / 2 = 171):